The family of the great train robber Ronnie Biggs today urged the justice secretary, Jack Straw, to reconsider a decision to block his release.

In a surprise move yesterday, Straw rejected a parole board recommendation that Biggs be released, saying the 79-year-old was "wholly unrepentant" about his crimes.

Biggs's son, Michael, said the decision "beggars belief", adding: "I hope that Mr Straw finds it in his heart to review his recommendation not to release my father.

"My father represents no threat to society whatsoever. He has paid his debt to society – what more could they want?"

He contrasted his father's sentence with shorter jail terms handed out to other criminals, including the killers of Baby P, and said: "This is not justice."

Frail and in a wheelchair, Biggs has had a series of strokes and his health is failing. He broke his hip three days ago after falling out of bed.

He is in the Norfolk and Norwich University hospital, where friends say he is fed through a tube and can only communicate through an alphabet board.

His son said: "He cannot walk, he cannot talk, he cannot read or write, he cannot drink – how can he take any reoffending courses?

"If this is the British legal system, it is appalling, it's beyond comprehension."

Biggs was jailed for 30 years for his part in what remains the most well known robbery in British history: the £2.5m hold-up of the Glasgow to London train in 1963 in which Jack Mills, a train driver, was beaten unconscious.

Referring to Biggs's escape from Wandsworth prison a year into his sentence, Straw said: "Biggs took the personal decision to commit another offence and escape from prison, avoiding capture by travelling abroad for 35 years while outrageously courting the media.

"Biggs chose not to obey the law and respect the punishments given ... the legal system in this country deserves more respect.

"It was Biggs's own choice to offend, and he now appears to want to avoid the consequences of his decision.

"He is wholly unrepentant, and the parole board found his propensity to break trust a very significant factor. He has not undertaken risk-related work and does not regret his offending."

After he absconded, Biggs fled to Brazil, where he lived for 35 years and was photographed by newspapers suntanned, smiling and surrounded by women.

He returned to Britain voluntarily in 2001.

The Tory MP Ann Widdecombe led criticism of the decision. "The prisons are bursting at the seams," she said.

"The courts are being urged to let burglars go free, but one doddery and very frail old man is being kept in prison.

"If you have got a prison place, use it to lock up someone who is genuinely a risk to the public."

The parole board, which met earlier this month, had recommended Biggs's release and said he posed a "manageable" threat to the public.

It noted that he was unrepentant about fleeing prison and going on the run for 35 years, but the train robber's son and friends had hoped he would be released.

Responding to Straw's statement, Biggs's legal adviser, Giovanni Di Stefano, said the only reason the train robber had not taken part in work to assess his risk and prepare for his release was because he was so ill.

"All the other great train robbers served a third of their sentences – why should Ronnie Biggs be any different? Ten years is enough," he said.

"This shows a side of the British government that is perverse – it is cruel and unusual punishment."

The parole board is unlikely to look again at the decision for many months.